combat fatigue

ret lt col dave grossman wrote the book on combat and there he deals with the problems that face soldiers in extended stays in a combat zone. well, all of america is a combat zone, so this begs the question: how do you stay fighting fit and mentaly stable (cou-skittles-gh) when everyday you face the threat of death and are in a perpetual state of preparedness to enter mortal combat to defend yourself. i just started reading the book, but he talks about a 60 day period (i think, i'll look back at it when i get home) after which you're pretty much comatose. of course he is dealing with fighting a human foe and our culture has established that zombies are not human, so i don't know how that would change things, but you can't stay in combat forever without a break. thoughts?